This invention relates to a screw driver, more particularly to a highly efficient impact screw driver which is easily manufactured and assembled.
Referring to FIG. 1, when using an earlier conventional impact screw driver 1 made of metal, the operator grips the screw driver 1 with one hand, and holds a hammer 2 with another hand, so as to strike the end of the screw driver 1 with the hammer 2. It is difficult to simultaneously carry the heavy screw driver 1 and the hammer 2. To overcome this drawback, as shown FIG. 2, an improved impact screw driver has been developed which includes an impact hammer 1 which can move downward to compress a coiled spring 2. Then, the compressed spring 2 pushes the tool bit downward. Because the push of the spring 2 is not a direct force, the efficiency of the screw driver suffers.